The only clue I have for now is that when I connect the rows and columns to the multiplexers a voltage drop seems to occur in the phone (LED is much less brighter).

Many times I suspected something like "maybe the voltage from the matrix scanning is routed in a wrong way". Could it be sucked through the GROUND pin of the multiplexer ?!I also discovered something strange : COMMON IN/OUT of the multiplexer are shorted with +5V of the arduino. Don't know why...I think there is some electronic trick that i don't understand here.